In order to simplify chemical carcinogenesis so that mechanisms of carcinogenesis can be studied in a defined environment at the cellular level in the absence of nerve supply, unknown hormonal influences, and undefined nutritional influences, recent investigations have focused increasingly on in vitro carcinogenesis. Most successful in vitro carcinogenesis has been performed using complex mixtures of embryo cells or other cell mixtures and has resulted in the production of a preponderance of sarcomas. In our laboratory, we have developed methods for obtaining single kinds of highly purified, viable mammalian cells which have been cultured following purification. We propose to use chemical carcinogens to transform highly purified mammalian cells which have been purified previously in our laboratory. Use of purified cells should result in the ability to produce specific kinds of tumors from specific kinds of cells in the absence of the confusing multitude of unknowns associated with the use of complex mixtures of cells. For the first time, it should be possible to produce carcinomas from specific kinds of epithelial cells without first exposing the target cells to prolonged tissue culture. The use of known cell types will also facilitate our designation of specific chemical carcinogens as proximate carcinogens or ultimate carcinogens for specific cells since the microsomal oxidases of undesired cell types can be excluded from the system. Transformed epithelial cells will not be overgrown by transformed fibroblasts since fibroblasts will not be present in the target cell population.